Adeno-associated virus (AAV) usually is defective for replication and depends on co-existent adenovirus or herpesvirus infection for efficient replication and a productive life cycle. In the absence of helper virus, AAV can undergo stable integration of its genome into the host cell but the integrated AAV genome has no pathogenic effect. These properties formed the basis for the development of AAV vectors for gene expression in mammalian cells. AAV vectors have been used to express both selective markers (Hermonat and Muzyczka, 1981, Proc. Natl. Acad. Sci. USA 81:6466-6470; Tratschin et al., 1985, Mol. Cell. Biol. 5:3251-3260) such as neomycin phosphotransferase (neo) and unselected genes including chloramphenicol acetyltransferase (cat) (Tratschin et al., 1984, Mol. Cell. Biol. 4:2072-2081) and thyroid stimulating hormone in eukaryotic cells (Mendelson et al., 1988, Virology 166:154-165; Wondisford et al., 1988, Molec. Endocrinol. 2:32-39).
For use as a viral transducing vector AAV may present some advantages including a high frequency of stable DNA integration and the lack of pathogenicity of wild type AAV. One limitation of AAV is that of size, since the packaging limit for foreign DNA in AAV particles is approximately 4.5 kilobases. This limitation is an important consideration for the design of AAV vectors for expression of genes or cDNA constructs in which the gene coding sequence approaches that of the AAV packaging limit, i.e., approximately 4.5 kilobases.
One such gene, for example, is the cystic fibrosis gene (CFTR). The airway epithelium is a critical site of cellular dysfunction in cystic fibrosis (CF), the most common lethal genetic disease in North America, and is characterized by a defect in regulation of Cl.sup.- conductance (Hwang et al., 1989, Science 244:1351-1353; Li, et al., 1988, Nature (London) 331:358-360, Li et al., 1989, Science 244:1353-1356; Schoumacher et al., 1987, Nature (London) 330:752-754). The cDNA for the CFTR gene (Riordan et al., 1989, Science 245:1066-1073; Rommens et al., 1989, Science 245:1059-1065) has been expressed in eukaryotic cells. Expression of the CFTR protein in non-epithelial cell lines resulted in generation of a Cl.sup.- conductance (Andersen et al., 1991, Science 251:679-682; Kartner et al., 1991, Cell 64:681-691). The CF defect has been complemented by expression of CFTR in a CF pancreatic adenocarcinoma cell line by stable transduction with a retrovirus vector (Drumm et al., 1990, Cell 62:1227-1233), and in a CF airway cell line by infection with a vaccinia virus (Rich et al., Nature (London) 347:358-363) or an adenovirus vector (Rosenfeld et al., 1992, Cell 68:143-155).
Gene therapy has been proposed as a way to reverse the cellular defect and prevent progression of disease in affected patients. Previous approaches to gene therapy have involved in vitro transduction of cells (such as lymphocytes) which can be easily reintroduced into patients. This may be difficult in an intact respiratory epithelium. An alternative approach is to use a virus vector to deliver the CFTR gene directly to the airway surface. One candidate is adeno-associated virus (AAV), a human parvovirus. The coding sequence (Riordan et al., 1989, Science 245:1066-1073) of CFTF, however, is 4.4 kilobases, which approaches the packaging limit of AAV particles. Thus, AAV has a potential drawback for its use as a vector for CFTR in that it barely accommodates the coding sequence of CFTR (Collins, 1992, Science 256:774-779).
AAV transducing vectors are described in the patent of Carter et al., (U.S. Pat. No. 4,797,368, issued Jan. 10, 1989). This patent describes AAV vectors using AAV transcription promoters P.sub.40, P.sub.19 and P.sub.5.
AAV vectors must have one copy of the AAV inverted terminal repeat sequences (ITRs) at each end of the genome in order to be replicated, packaged into AAV particles and integrated efficiently into cell chromosomes. The ITR consists of nucleotides 1 to 145 at the left end of the AAV DNA genome and the corresponding nucleotides 4681 to 4536 (i.e., the same sequence) at the right hand end of the AAV DNA genome. Thus, AAV vectors must have a total of at least 300 nucleotides of the terminal sequence.
For packaging large coding regions, such as the CFTR gene into AAV vector particles, it is important to develop the smallest possible regulatory sequences, such as transcription promoters and polyA addition signal. Also in this latter study and another study (Beaton et al., 1981, J. Virol. 63:4450-4454) it was shown that the AAV ITR sequence can act as an enhancer for the SV40 virus early gene transcription promoter. However, it was not shown that the AAV ITR region had any intrinsic transcription promoter activity. Indeed, it is taught in the literature that the AAV ITR regions have no transcriptional function (Walsh et al., 1992, PNAS [in press]). Therefore, in the previous AAV vectors a small transcription promoter was utilized, namely the AAV P.sub.5 promoter, which consists of nucleotides 145 to 268 of the AAV genome positioned immediately adjacent to an ITR.
Thus, there exists a need to increase the packaging size of AAV while maintaining efficient expression. The present invention satisfies this need by showing that a gene can be functionally expressed from an AAV vector when it is positioned adjacent to the AAV ITR even in the absence of another promoter. This finding demonstrates a previous unrecognized ability of AAV termini to function as fully competent transcription promoters. This demonstrates that AAV vectors can be constructed in which the ITR itself is acting as the transcription promoter and no other promoter sequences must be incorporated into the vector. It is also shown that a CFTR fusion gene consisting of the CFTR cDNA and a synthetic oligonucleotide positioned in an AAV vector immediately adjacent to an AAV ITR can be functionally expressed in human cells to correct the cystic fibrosis defect.